Previously, steel sheets produced by disposing metal plating containing zinc or aluminum on surfaces of steel sheets and conducting a chromate treatment have been widely used as steel sheets for automobiles, steel sheets for household electrical appliances, and steel sheets for construction materials. The chromate treatment is a treatment to form a chromate layer by using a treatment solution containing hexavalent chromium as a primary component for the purpose of improving the corrosion resistance (white rust resistance, red rust resistance). However, hexavalent chromium that is a controlled substance to prevent the pollution is used in the chromate treatment. Recently, controls on the use of chromium has been stepped up from the viewpoint of the degree of influence on the environment.
Consequently, the surface treatment is directed to become chromate-free. Particularly in consideration of the influence exerted on a waste water treatment during manufacture of a surface treated steel sheet or operators and users who handle products, an environment-conscious surface treated steel sheet has been required in order that the product contains no substance (in particular, hexavalent chromium) harmful to the environment and the human body. Although Cr3+ exerts a smaller influence on the environment and the human body as compared with an effect of Cr6+, it is difficult to distinguish Cr6+ from Cr3+ in a film when these are used commercially in practice. Under these circumstances, demands for a chromate-free film containing no chromium compound and having outstanding performance, e.g., corrosion resistance comparable to the chromate treatment, have been intensified.
In order to meet up the above-described demands, many technologies have been proposed, in which thin films are formed on plating surfaces by methods of immersion, coating, an electrolytic treatment, and the like through the use of solutions containing vanadium compounds as components alternative to hexavalent chromium in the chromate treatment solution which has been used previously.
In particular, a vanadium compound having a valence of five has been noted, the vanadium compound widely known as a rust inhibitor of zinc and aluminum. Since the pentavalent vanadium compound has a noble oxidation-reduction potential and exhibits an oxidative effect, it is believed that the pentavalent vanadium compound forms a passive film on a plating surface and exert an antirust effect as an inhibitor of an anode corrosion reaction in a corrosive environment.
For example, Japanese Unexamined Patent Application Publication No. 1-92279 and Japanese Unexamined Patent Application Publication No. 1-31281 disclose methods in which a treatment is conducted with a paint primarily containing phosphoric acid ions and vanadic acid ions. These methods are intended to use primarily the antirust effect of the pentavalent vanadium compound. However, a large antirust effect is not exerted in practice. In addition, a surface treatment solution containing a vanadium compound and a zirconium compound, a titanium compound, or the like has been proposed in Japanese Unexamined Patent Application Publication No. 2002-30460. In this technology, a film is formed by divalent to tetravalent vanadium compounds prepared by partial reduction of pentavalent vanadium compounds. However, in this case, satisfactory corrosion resistance cannot be achieved since divalent or trivalent compound films having a small corrosion-inhibiting effect are contained simultaneously. Furthermore, when exposed at a humid environment, the film is significantly colored and the appearance is deteriorated.
On the other hand, from the viewpoint of application of the above-described surface treated steel sheet to, for example, construction material, e.g., roofing materials and outside wall materials, or household electrical appliances, e.g., back boards of refrigerators, most of all, a high-Al-containing Al—Zn alloy-plated steel sheet and the like typified by Galvalume (55% Al-1.5% Si-remainder Zn) are used in many cases. The reason therefor is that the Al—Zn alloy-plated steel sheet has beautiful plating appearance and exhibits excellent corrosion resistance and, thereby, it can be used without painting in every application. In these applications, the plated steel sheet is required to exhibit corrosion resistance for the long term and, in addition, it is also required that the plating surface is not discolored by exposure to a humid environment and the like, and the commercial value resulting from the beautiful appearance of the plating surface is maintained for the long term.
Furthermore, when the metal plating containing zinc or aluminum is subjected to working, a plating film is peeled off, and the appearance is deteriorated significantly. Consequently, the surface treated steel sheet may be required to have excellent workability. Since the adhesion to the plating film is indispensable to improve the workability, production of a low solubility film by using an inorganic component is effective in improving the workability. In the application to construction materials, a plated steel sheet is molded by roll forming. Therefore, it is required that the plating is not picked up on a roll (that is, good roll forming property is exhibited). In the application to household electrical appliances, a required property is that the appearance after press molding is not blackened due to sliding over the mold.
However, as shown in, for example, Japanese Examined Patent Application Publication No. 1-53353, Japanese Examined Patent Application Publication No. 4-2672, and Japanese Unexamined Patent Application Publication No. 6-146001, a surface treatment film containing an organic resin and a hexavalent chromium compound is formed on a plating surface to meet up the above-described applications under present circumstances. Therefore, a film containing no chromium, that is, a so-called chromium-free film, has not been achieved.
Against such a background, a technology of combining a vanadium compound and an organic resin has been proposed.
For example, a method for forming a coating containing an organic resin, a thiocarbonyl-containing compound, and a vanadium compound is disclosed in Japanese Unexamined Patent Application Publication No. 2000-248380. In this method, the thiocarbonyl-containing compound exerts an effect of improving the corrosion resistance, but a primary component, that is, pentavalent vanadium compound, does not contribute significantly to the improvement of the corrosion resistance. Furthermore, a treatment with a surface treatment agent containing a specially modified phenol resin, a vanadium compound, and a metallic compound of zirconium, titanium, or the like is disclosed in Japanese Unexamined Patent Application Publication No. 2001-181860. In this technology, the specially modified phenol resin exerts a largest effect on the corrosion resistance, an effect of a metal salt of vanadium, zirconium, or the like is small and, therefore, the corrosion resistance cannot be said satisfactory as compared with that of the chromate treatment film.
As described above, each technology exhibits the corrosion resistance to some extent but exhibits a poor oxidizing power as compared with that of the chromic acid ion. Therefore, the corrosion resistance is not satisfactory as compared with that of the chromate film. If the amount of adhesion is increased to ensure the corrosion resistance, in the case where the film contains the pentavalent vanadium compound, there is a problem in that the appearance after drying takes on a yellow tinge derived from the pentavalent vanadium compound. In addition, when exposed to a humid environment, films of vanadium compounds other than the pentavalent vanadium compound are also colored and, therefore, the appearance quality and the corrosion resistance of the film have not been able to become compatible with each other.
It could therefore be helpful to provide a surface treated steel sheet containing chromium in a film and exhibiting excellent, corrosion resistance and surface appearance, as well as a method for manufacturing the same. Furthermore, a surface treated steel sheet exhibiting excellent workability in addition to the corrosion resistance and the surface appearance, as well as a method for manufacturing the same, is provided.